This image taken from video obtained from Ugarit News on Friday, Feb. 8, 2013, which has been authenticated based on its contents and other AP reporting, show smoke rising from fighting near a main highway in Damascus, Syria. Rebels pushed forward in their battle with the Syrian army in northeastern Damascus on Friday, shutting down a main highway with a row of burning tires, activists said. A number of rebel brigades launched a campaign Wednesday to attack regime checkpoints along the highway and have been clashing in the area since. The government has responded by shelling number of rebel areas nearby.

BEIRUT — Rebels pushed forward in their battle with the Syrian army in Damascus on Friday, clashing with regime soldiers in contested neighborhoods in the northeast and shutting down a key highway out of the capital with a row of burning tires, activist said.

In Geneva, the U.N. refugee agency said there has been a huge increase in the number of people fleeing Syria, with 5,000 refugees crossing the borders daily into neighboring countries. Agency spokesman Adrian Edwards said Friday the mass exodus “is really a full-on crisis right at the moment.”

The latest fighting in Damascus, some of the heaviest to hit the city since July, began Wednesday with a series of rebel attacks on regime checkpoints along the main road from Damascus to northern Syria. Opposition fighters and government forces have been clashing in the area since, and regime troops have also responded by shelling a number of rebel-held districts nearby.

The violence has brought the civil war that has destroyed entire neighborhoods of other Syrian cities closer to the heart of the capital, which has mostly been spared heavy fighting. Still, the offensive did not appear to be coordinated with rebels on other sides of Damascus and it was unclear whether the rebels would be able to hold their ground.

Both the rebels and the regime of President Bashar Assad consider the fight for Damascus the most likely endgame in a civil war that has already killed more than 60,000. The government controls movement in and out of the heavily defended city with a network of checkpoints, and rebels have failed so far to make significant inroads.

A spokesman for one of the opposition groups fighting in the area said the rebels sought to open a path for a future assault on the city.

“This is not the battle for Damascus. This battle is to prepare for the entry into Damascus,” he said via Skype, giving only his nickname of Abu al-Fida for fear of reprisals.

The fighting revolved around the capital’s main highway heading toward the country’s north. Abu al-Fida said one checkpoint on the highway changed hands twice on Thursday but was securely in rebel hands Friday. He said rebels were within a half-mile from Abbasid square and were firing mortars at a military base near the landmark plaza.

Online videos showed a row of burning tires laid across the highway, blocking all traffic. Smoke rose from a number of areas nearby, reflecting clashes and government shelling.

The videos appeared genuine and corresponded to activist reports.

The Britain-based Syrian Observatory for Human Rights reported clashes in Jobar and shelling and airstrikes on the nearby areas of Zamalka and Qaboun. Rebels also battled government troops in the southern neighborhood of Yarmouk, as well as in the rebel-held suburbs of Daraya and Moadamiyeh, where six people died in a government shell attack, it said.

Also Friday, the Observatory said 54 were killed, including 11 women, in a bombing at a bus stop near a military factory earlier in the week.

Observatory director Rami Abdul-Rahman said an explosive-laden mini-bus blew up at a bus stop near the factory in Buraq, near the central city of Hama, while workers were waiting for rides home. The factory makes military supplies, but not weapons, he said.

The area is government-controlled, which is why reports on the blast were slow to emerge.

“These people work for the Ministry of Defense, but they are all civilians,” he said, adding that no one from the military was killed in the blast.

Facebook pages for nearby villages posted names of the dead and pictures of mass graves. A page for the nearby town of Salmiyeh listed more than forty residents it said were killed in the blast.

Syria’s state news agency reported the explosion on Wednesday evening, saying “terrorists” detonated a car bomb near a factory. It did not say what the factory produced or specify the number of dead and wounded. The regime refers to rebels fighting to topple the Assad regime as terrorists.

No one has claimed responsibility for the blast, which resembled others in recent months that appeared to target buildings associated with Syria’s military and security services.

Some of the bombings have been claimed by an al-Qaida-linked group fighting alongside the rebels, Jabhat al-Nusra, which the U.S. had designated a terrorist organization.

As the situation in Syria has worsened, foreign jihadists have flocked to Syria to join what they consider a holy war to replace Assad’s regime with an Islamic state in Syria. Most of the foreign fighters are Arabs from Egypt, Tunisia, Libya and other countries.

Late Thursday, the chief of the Netherlands’ top intelligence agency warned that dozens of Dutch citizens are fighting with Syria’s rebels and could return home battle-hardened, traumatized and radicalized.

General Intelligence and Security Service chief Rob Bertholee told the Dutch show Nieuwsuur that hundreds of people from around Europe and dozens from the Netherlands have travelled to Syria to join rebels fighting Assad.

He said propaganda romanticizing the civil war is helping draw foreigners into Syria’s maelstrom of violence.

Syria’s crisis began with peaceful protests in March 2011 and evolved into a civil war as the opposition took up arms to fight a government crackdown on dissent. The U.N. said last month that more than 60,000 people have been killed in the conflict.